January 22, 2018 - To ensure optimal performance of the LSST telescope, a regular cleaning schedule of its mirrors will be implemented during operations. Weekly, the Primary/Tertiary Mirror (M1M3) and Secondary Mirror (M2) surfaces will be cleaned by spraying the surfaces with a carbon dioxide (CO2) “snow” which removes dust particles and other contaminants. Approximately every six months, M1M3 will be cleaned with neutral soap and water using very soft chamois mops, followed by de-ionized water rinsing and drying using small air knives around the mirror cell. Both of these cleaning procedures take place without removing the mirrors from the telescope, and are completed within one summit working day. Neither the CO2 nor wet contact wash will impact the nightly observing schedule.
In addition to these preventative maintenance activities, the mirrors will be stripped of their reflective coatings, washed, and recoated every few years. We anticipate the M1M3 (coated with aluminium) will be cleaned and recoated every 2 years, and the M2 (coated with protective silver) every 5 years. Reflectance monitoring will allow us to predict when this more time-intensive work is necessary, and it will likely be coordinated with other scheduled downtime.
The washing station, which provides for pristine optical surfaces before coatings are deposited, is a deliverable part of the coating plant contract. A Provisional Acceptance Test of the washing chamber will take place later this month at the Von Ardenne facilities in Dresden, Germany. The review includes observation of the washing boom, seen in this 30-second video provided by LSST Coating Chamber Engineer Tomislav Vicuna. This special machinery will remove the old mirror coating and wash the mirror prior to moving it into the coating chamber for recoating.
A related story about the LSST coating chamber, from March, 2017, can be found here.